Use of polarization for differentiation of information

ABSTRACT

The present invention discloses a system for signalling within optical or combined optical/electronic networks wherein a first transmission node executes polarization multiplexing on transmitted traffic, and at one or more intermediate nodes one or more of the following processes are carried out on the sent traffic: demultiplexing by polarization and/or polarization and/or SOP-alignment. Further a method for packet handling within optical packet switched networks where, at a first transmission node carries out polarization demultiplexing of transmitted traffic, and at one or a number of intermediate nodes carries out one or more of the following processes on the transmitted traffic; demultiplexing by polarization, and/or polarization and/or SOP-alignment. Said separation into states of polarization is used in separation of QoS-classes.

FIELD OF THE INVENTION

The present invention relates to polarization to distinguish QoSclasses, and to distinguish payload and header in packets withcommunication networks. More generally the present invention relates toa new and improved use of states of polarization within all types ofcommunicational networks.

BACKGROUND OF THE INVENTION

With the introduction and the development of optical networks it is agoal to reduce the cost and complexity of data transmission within voiceand data networks. A major factor for achieving this is to reduce thenumber of signal transformations between optical and electrical signals.Such a reduction will reduce the number of components within thenetworks elements and reduce the need for electronic signal processing.Further a reduction in the number of components within the networkselement will result in a reduction of the sources of errors, and hencereduced need for service and maintenance and an increased operationaltime. These factors will again result in a potentially reduced cost.

The traffic volume of Internet is reported to show a significantincrease despite the downturn of the telecommunication industry. Hence,ingreasing parts of the traffic in the transport network origins frompacket data. For obvious economic reasons, new switching techniquesshould first be introduced at the time they show maturity and costeffectiveness. Hence there is a need to develop flexible opticalnetworks supporting a seamless migration from an optical circuitswitched (OCS) to an optical packet switched (OPS) backbone network.

Thus replacing electronical network element with optical networkelements it is necessary that the optical network elements have afunctionality which can operate effectively within a packet switchednetwork. In the last few years intensive research have been spent onoptical packet switching (OPS), and optical burst switching wherepackets or bursts of packets are switched directly in the optical layerwith optical switches. These techniques are expected to be commerciallyof interest within approximately four years.

The Five Dimensions

As optical signal processing is still immature there are very restrictedpossibilities for signalling different types of information such asaddress information. Dimensions available for transfer of information inan optical fibre are: intensity, time, frequency, phase andpolarization. All these dimensions are through the years suggested usedfor different purposes.

The formats of modulation used in optical links and networks are todaybased on NRZ- and RZ-formats where intensity varies between a minimum-and maximum level. The signals are time divisional multiplexed (TDM)with a data rate between 2.5 and 40 Gb/sec. In optical line switchednetworks the available and useable optical frequency spectrum is usedfor multiplexing a number of TDM-channels within one fibre, so calledWavelength Division Multiplexing (WDM). The optical frequency is alsosuggested used as a label with optical networks where the framework fromMPLS is used. Phase and frequency are suggested used as a form ofmodulation as to increase spectral efficiency likely in combination withpolarization.

Optical Package Switching, Address, QoS and Signalling.

In connection with optical package switching transfer of addressinformation in the form of a header or a label is a problem fordiscussion. Normally, in an electronic router the header will betransferred at the beginning of the package or the frame, and theaddress information and payload is thereby time multiplexed.Demultiplexing in the time domain is difficult using optical components.Transfer of address information separated from payload is thereforesuggested carried out in different manners such as:

1a) Address and payload are separated by the use of separate opticalwavelengths; this gives however a bad utilization of the wavelengths.

1b) Usage of a separate frequency within the optical wavelength,so-called Sub Carrier Modulation (SCM), utilizing the optical wavelengthmore efficiently than when a separate wavelength is used. However, thissolution may lead to a deterioration of the payload signal.

1c) In the EU-sponsored project “STOLAS” it is suggested to usefrequency modulation for modulation of package header separated from thepayload; however this method may also give a deterioration of the signalquality within the payload. STOLAS is an ongoing project within EUs5^(th) general plan “IST”. Reference for this theme within the project:Sulur, T. K. et al. “IM/FSK Format for Payload/orthogonal Labelling IPPackets in IP over WDM Networks Supported by GMPLS-based LOBS.” ONDM2003, Feb. 3-5, 2003, Budapest, Hungary.

Several techniques have been proposed for in-band header encoding, likeserial header, SubCarrier Modulation (SCM), and Frequency Shift Keying(FSK). However, they require advanced components for separation ofheader and payload, and reinsertion of new headers. To erase old header,before a new one can be inserted, per input wavelength, serial headerrequires a fast optical gate e.g. a Semiconductor Optical Amplifier(SOA), while SCM and FSK need an optical wavelength converter. Thisincreases component count of complex and yet technologically immaturecomponents. Furthermore, if separation of packets belonging to differentQoS classes is desirable, it will normally be done based on electronicprocessing of the header information, hence not all-optical.

Known Principles

1) Use of polarization of multiplexing/demultiplexing of two datachannels (multiplexing by polarization) on one fibre is a knownprinciple.

2) Use of polarization to find a start and stop on a bit-sequence isknown, consequently by changing state of polarization.

3) To separate different optical data channels by polarization in thesame manner as different optical data channels may be separated on itswavelength. Optical ad/drop entities based on separation betweenorthogonal polarizations like similar entities which are distinguishedby the use of wavelength demonstrated and referred to in the literature.

4) Separation of header- and payload by the use of polarization ispatented.

Introduction

In a statistical multiplexed packet switched network, services likeconstant delay, and no packet loss, can not be guaranteed due to thevery nature of statistical multiplexing. This may preclude the use ofstrict real-time applications, where delay is critical, and packet lossshould be at an absolute minimum, like e.g. for remotely controlledsurgery. Guaranteed service (GS), without contention causing packetloss, and a fixed delay, can however be offered if the packets are sentthrough a network following a path with pre-assigned resources, like ina Static or Dynamic Wavelength Routed Optical Network (S-WRON orD-WRON). D-WRONs increases throughput efficiency, compared to S-WRONs,by dynamically reconfiguring the wavelength paths to adapt to thetraffic demands. However, the control plane operates on an ms to stimescale, and cannot be optimized to the bursty traffic patterns ofOPS, where packet durations are typically in the μs range. Therefore,not even D-WRONs can achieve the throughput efficiency and granularityof statistical multiplexing.

The Package Switch

A package switch may be partial optical and partial electronic or fullyoptical.

In EP 07944684 A1 it is described an optical package switched networkwith one or several nodes and a transmitter sending polarized packagesignals. The package signals comprising a header- and a payloadseparated from each other by way of orthogonal polarizing. Further it isknown from CA 2352113 an optical method of communication where it isutilized a high speed polarized bit stuffing method. The methoddescribes a way of using polarized bit-stuffing for separation of datapackage instead of multiplexing data streams from different modulators.This increases the speed for transferring of data within an opticalnetwork.

Optical packet switching (OPS) is promoted as a way to overcome theelectronic bandwidth bottleneck. However, if OPS nodes are to berealised, they must also prove to be cost effective. The presentinvention proposes to use polarisation multiplexing for a low-costseparation and reinsertion of control information in OPS, as well as foroptical differentiation between Quality of Service (QoS) classes. Thetwo applications can be performed simultaneously or separately.

In the present invention it is proposed to combine the properties of astatistically multiplexed packet switched network (OPS) with the GSenabled by optical circuit switched networks (like S-WRON/D-WRON) in asingle optical network layer. This requires that the circuit switched GSpackets and the OPS packets efficiently share the data layer resources.A node design that allows full sharing of link bandwidth is proposed,and that allows a migration from an S-WRON to the more efficientcombined network, by adding OPS capability. The efficiency of the nodeis studied using a simulator.

The technique proposed here, as presented in the present invention,overcomes the drawbacks as described above by using orthogonal States ofPolarisation (SOP) for separating packets and sending controlinformation. By using a Polarisation Beam Splitter (PBS) per wavelengthfor header/payload separation, the complexity and cost may be reducedsignificantly, compared to the solutions mentioned above.

BRIEF DISCUSSION OF THE INVENTION

The present invention is trying to avoid the problems mentioned earlierwhich are linked to today's solutions, since it presents a method and asystem for signalling within optical or combined optical/electricalnetwork characterized in that one at a first transmission node executespolarization multiplexing of transmitted traffic, and that at one ormore intermediate nodes is executing one or more of the followingprocessing of the transmitted traffic:

demultiplexing of polarization of the received traffic and/or

multiplexing by polarization and/or time divisional multiplexing of thereceived traffic, and/or

SOP-alignment of the received traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

The enclosed drawings which are included and which form a part of thespecification are illustrating embodiments of the present invention andserves, together with the description, as an explanation for theprincipals of the invention.

FIG. 1 shows packages which belong to different QoS-classes assignedrelative orthogonal states of polarization. Thus it becomes possible forthe receiver to separate two classes of priority optically by the use ofa simple polarization beam splitter,

FIG. 2 shows the proposed node design. The resources used in the1-switch and packet switch are shared. The number of inputs neededequals the number of input fibres X the number of link-wavelengths,GS=Guaranteed Service. &=Optical And Gate,

FIG. 3 shows experimental setup. PC=Polarisation Controller, i.e. devicefor adjustment of state of polarisation.

FIG. 4 shows, Sensitivity curves for two signals, both for back-to-back(stippled lines) and at the egress node. The characteristic of thetransmission for the header/payload separation is measured usingmodulation on both transmitters, thus this is the most criticalsituation, with crosstalk between the two polarisation signals.Modulation of both transmitters at the same time is equal to sendingheader- and payload simultaneously, or having two packets with differentQoS class transmitted at the same time. Using this principle prohibitsthe use of the node as shown in FIG. 2 due to the fact that this nodedepends on simultaneous modulation of only one of the polarising stateswhere the other is used as a control signal. Experimentation withmodulation of one transmitter at a time has shown less signaldeterioration. This equals sending header- and payload serially andhaving packets belonging to different QoS being sent serially. The nodeas shown in FIG. 2 is supporting this principle. A polarisation state isthe modulated whereas the other serves as a control signal so as toindicate whether a header is transferred to the BE-packet or to a GSpacket.

FIG. 5 shows, the OPS part of the node as an embedded part of an S-WRONnode with a hardwired cross coupling matrix. In the S-WRON configurationexample, each of the nodes connected to the inputs have ‘k’wavelength-connections available to each of the outputs. The fixedwavelength converters enable on-line reconfiguration of the S-WRON.FDL=Fibre Delay Line, n=number of link-wavelength. N=Number of linkinputs,

FIG. 6 shows a PLR for a system with exclusive BE-traffic for 32λ aswell as with GS-shares of 10% and 30% at 50 kb and 10 and 50% at 500 Bpacket lengths,

FIG. 7 shows a PLR for a system with exclusive BE-traffic for 128λ aswell as with GS-shares of 10% and 30% at 50 kb and 10 and 50% at 500 Bpacket lengths

FIG. 8 shows a sketch of a hybrid switch. The router may handle two QoSclasses, best effort (BE) and a first priority traffic. The polarizationof the light is used for signalling the QoS class. BE is electronicallymanaged, whereas first priority traffic are optically handled. In theoptical switch it is the wavelength that determines where to forward apacket,

FIG. 9 shows a scalable design with QoS priority. Whether the packetwill have priority or not, is decided by the state of polarisation ofthe packet at the input,

FIG. 10 shows, a network with a number of nodes and QoS connectionsbetween some of these,

FIG. 11 shows a reference scenario for a pan-European network with 37nodes, with a maximum node degree of 5.

DETAILED DESCRIPTION OF THE INVENTION

In the following it is given a detailed description of the presentinvention with support in the enclosed figures. As mentioned earlierthere are several problems related to OPS. The present invention isredressing these problems by other things by using polarization forsignalling.

One can imagine the signalization being used in different ways:

3a) Synchronizing, the state of polarization changes at the beginning ofeach package.

3b) Header- and payload within each package is separated by orthogonalstates of polarization.

3c) QoS-classes are separated by assigning different priority topackages to be processed, thus having different polarization on thetransmitter side.

In FIG. 1 it is given an example on how the states of polarization maybe used for optical separation between two different QoS-classes. Thesame principle may be utilized to separate optically between a header-and a payload. The method may, with the use of a polarization beamsplitter, separate the information dependent of the wavelength. If aWDM-signal with a number of wavelengths is sent towards the splitter,the splitter will function as a demultiplexer for header- and payload orQoS-classes for all the wavelengths.

FIG. 2 illustrates one embodiment of the present invention. Header andpayload separation is implemented by sending the header in an SOPlabelled ‘1’, and the payload in SOP ‘2’, orthogonal to ‘1’. Separationis done using a PBS, allowing full transparency with respect to bit rateand signal format for both header and payload.

Additionally, if a very high QoS is needed with a Guaranteed Service(GS) with respect to packet loss and delay, like e.g. remote imageguided surgery, the GS packets may be forwarded solely on the basis oftheir wavelength information using a wavelength router. These packetscan be separated from e.g. Best Effort (BE) packets by transmitting BEpackets in SOP ‘2’, while GS packets are transmitted in the SOP ‘1’,like in FIG. 2. GS packets will then pass through a wavelength routednetwork allowing GS, while BE packets will be interleaved with the GSpackets at the output of each node, increasing the utilisation of thelinks. The GS-packages are delayed equally to the longest BE-package inevery node so that by detecting a GS-package on the input one canreserve the output and make sure that for the moment no BE-package istransmitted. In this way package contentions between BE- and GS-packagesare avoided.

Both described embodiments can be combined. GS packets will then be sentin SOP ‘1’, without an orthogonal polarisation header, while BE packetswill be sent in the SOP ‘2’ with a simultaneously transmitted header inSOP ‘1’. When a signal is observed in SOP ‘1’, with a signalsimultaneously present in SOP ‘2’, the signal in SOP ‘1’ is recognisedas the header of a BE packet. If there is no signal simultaneouslypresent in SOP ‘2’, the signal is recognised as a GS packet. When usingthis method, detection of the simultaneous presence of signals in thetwo SOP's enables distinction of GS packets and BE headers. If serial BEheader is used, distinction can be implemented sending the signals fromthe two SOP's into an optical AND gate. The GS packets in SOP ‘1’ areforwarded through the AND gate if SOP ‘2’ is high, while if SOP ‘2’ islow, a BE header in SOP ‘1’ is present, and blocked by the gate.

If only one application is implemented, BE and GS packets, or a headerand a BE packet, can be sent simultaneously in both SOPs. This has thepotential of doubling the link's bandwidth utilisation.

AN EXEMPLIFIED EMBODIMENTS OF THE PRESENT INVENTION

The transmission properties of the principle have much in common withpolarisation multiplexing: Depending on the fibre's birefringence, PMDand the link distance, signals are depolarised. However, unlikeconventional polarisation multiplexing, where a polarisationdemultiplexing is done only at the receiver node, the present inventionincludes polarisation demultiplexing, polarisation, SOP re-alignment andpolarisation multiplexing in all intermediate “core nodes”. Thisincreases tolerance to depolarisation and changes in SOPs, therebyallowing longer transmission distances.

The quality of the signal path through a network model using one of thedescribed embodiments is shown. The experimental set-up, correspondingto a network consisting of an ingress node, a core node and an egressnode, is shown in FIG. 3. Two optical transmitters on the samewavelength are modulated at 2.488 Gb/s using two separate anddecorrelated bit generators with PRBS of length 2¹¹−1. The signals arecombined using a polarisation maintaining coupling (PM) coupler, andamplified using an EDFA. After the first 25 km SMF link, the signalsarrive at the “Core Node”. A manual Polarisation Controller (PC) ensuresan ideal SOP to allow optimum splitting of the two signals in a PBS. Toemulate forwarding of the signals, the two arms are combined using a PMcoupler, and sent to the receiver node, through the second 25 km SMFlink. Here, the two signals are again realigned and separated, beforesent to the receiver. Power penalties are found comparing transmissionpath and back-to-back Bit Error Rate (BER) curves in differentconfigurations. Because of the polarisation variations occurring in thefibre due to variations in the fibres environmental conditions, liketemperature variations, SOPs at the PBS inputs must be continuouslymonitored and optimised. The frequency of the variations caused by theenvironment is normally lower than an Hz; automatic polarisationoptimisation can therefore be used.

Two different transmission schemes, illustrating two embodiments weretested. The transmission characteristics of the header-payloadseparation is measured using modulation on both transmitters, whilesegregation of packets belonging to different QoS classes is done bymodulating only one transmitter at a time, leaving the other in CW mode.The most critical application is when both polarisation states aremodulated. However, as shown in FIG. 4, segregating packets wasdemonstrated with a very moderate penalty. The experimental points areinterconnected by a linear fit, taking all but the very last measurementof the two egress node series into account. These are omitted because,during the measurement, drift in absolute SOP's, thus sub-optimumheader-payload separation, occurred. This can be avoided using automaticPC. As shown in FIG. 4, a maximum penalty of less than 0.5 dB can beobserved at a BER of 10⁻⁹.

Thus the use of polarisation multiplexing for header and payloadseparation and for optical QoS differentiation in optical packetswitched networks has been shown. The principle of a packet switch nodeis described and the quality of the signal path through a network modelis experimentally verified. Using this principle, the need for complexcostly components is significantly reduced, and optical QoS packetsegregation is achieved. A maximum penalty of only 0.5 dB was observedat the receiver node after passing the suggested optical packet switchnode and 2×25 km of SMF.

Best Mode of a Node Design

The proposed node design according to the present invention is shown inFIG. 5, where an OPS module is added to an S-WRON node. Packets aredivided into two classes: “GS”, that follows the pre-assigned S-WRONpath, and a Best Effort class, “BE”, without service guarantees, whichis switched using the packet switch module. At the input, the two packetclasses are segregated by setting 1×2 switches based on information in aheader, or as shown in the FIG. 5 by using orthogonal States ofPolarization (SOP). Then each of the polarization states is assigned toeach of the service, classes.

Since the GS packets destinations are decided by the configuration ofthe cross coupling matrix and the individual wavelengths of the packets,as shown in FIG. 5, GS headers are superfluous. Since service classsegregation is achieved using SOP, contrary to principles as known fromthe prior art of optical QoS separation using reservation of wavelengthsin an OCS network, the wavelength domain can be entirely devoted towavelength routing purposes.

If a GS packet arrives at the switch, the control electronics registerthat a packet is present at the input before the packet is delayed in aFDL corresponding to the duration of a maximum sized BE packet,D_(BEMAX). The output for which the packet is scheduled is thenreserved. If a BE packet is currently propagating through the reservedoutput, it will not collide with the newly arrived GS packet, because ofthe delay in the FDL. Alternatively, the output can be reservedD_(BEMAX) in advance of the GS packet arrival by sending a controlpacket. If SOP is used for segregating the packet classes, the controlpacket should be sent in the same SOP as the GS packet, enablingsimultaneous transmission with potentially earlier transmitted BEpackets.

Performance Analysis

Packet delay and packet loss is found by simulation. Independent,asynchronous traffic generators, generating fixed length packets areused. The BE packets have a length of 500 B, while the GS packets lengthis varied, and set to either 500 B or 50 kB. An output is reservedD_(BEMAX) before a GS packet that arrives at the input enters theoutput.

The packets interarrival times are negative exponential distributed(Poisson), corresponding to a load of 0.8 of a maximum load on eachwavelength. Packet arrivals, both BE and GS, are uniformly distributedat the switch inputs. BE packet destinations are uniformly distributed,thereby also among the outputs. The GS packets are forwarded to a fixeddestination and wavelength, uniformly distributed, hence avoidingcongestion with other GS packets.

Electronic buffering is assumed, therefore the BE packets can stay inthe buffer for an arbitrary period of time. There is no limit on thesize of the buffer; however the registered maximum filling of the bufferwas 632 packets. The use of a very large buffer is therefore avoided.Buffered packets are normally scheduled as soon as a destined outputbecomes available, but reordering of packets may occur in rare cases.

The node performance is analysed in a transport network for 32 and 128wavelengths at a node-degree of 8, varying the number of buffer inputs.The maximum delay measured, was 0.21 times the packet duration, in thecase of 60 buffer inputs, 50% GS traffic share, 50 kB GS packet length,also giving maximum buffer filling. This is normally much lower than thetransmission delay and hence negligible. The simulation results showingPLR, for GS packet share of 10% and 50% of the total traffic load,measured in bytes, are shown in FIGS. 6 and 7, respectively.

In FIGS. 6 and 7 the performance with a system with only BE traffic iscompared. When the PLR for the pure BE traffic is 10⁻⁶, for 32wavelengths and a GS packet length of 500 bytes, the degradation is twodecades for a 10% GS traffic share, while for 50% GS traffic share thedegradation is more than three decades. If a GS packet length of 50 kBis used, the 50% and 10% curves are overlapping, and the degradation isone decade. FIG. 7, for 128 wavelengths, show the same tendency. At apure BE system PLR of 10⁻⁶, degradation can be observed for GS packetswhen the packet length is 500 Bytes. At a GS traffic share of 10%, thedegradation is approximately one decade, and for 50% GS traffic, thedegradation is approximately three decades. Worth noticing is that whenthe number of buffer interfaces is further increased, causing lowerPLR's, degradation is observed also when using GS packets of 50 kBlength. At a pure BE system PLR of 10⁻⁷, the degradation is one half ofa decade and one decade for 10% and 50% GS traffic shares, respectively.

FIGS. 6 and 7 illustrates that the overhead caused by the reservationtime is the main reason for the PLR degradation. Generally, adegradation increasing with the decreasing PLR and thus, number ofinterfaces, can be observed. When the GS packet length is large (50 kB),the degradation is low, independently of the GS traffic share. However,when GS and BE packets have equal length, the degradation may be severaldecades. From the simulation results we conclude that the PLR penalty islow if the GS packets are much longer than D_(BEMAX). Since GS packetsare given absolute priority, the very low BE PLR penalty observed, evenwhen 50% of the total traffic is GS packets, may come as a surprise.However, when increasing the number of GS packets in the system, thenumber of BE packets are decreased, causing less problems withcontention between BE packets and less load on the available bufferresources.

Conclusions

An OPS node design according to the present invention supporting GSwithout packet loss and with fixed delay, as well as a BE service classis proposed. The design supports a migration strategy from circuit topacket switching by starting with an S-WRON module and adding an OPSmodule. High capacity utilisation is obtained by interleavingstatistically multiplexed BE packets with GS packets that follow apre-assigned wavelength path. The penalty of introducing GS packets inthe system is shown to be very moderate if the GS packets is much longerthan the BE packets. Aggregation of GS packets into bursts musttherefore be considered.

Hybrid Electronic/Optical Switch with QoS

In the following section a hybrid construction is described where aswitch is built with an optical forwarding of a class of quality (GS)and an electronic forwarding of another class of quality (BE). FIG. 8presents a sketch of the building of the sketch.

The switch has two switching matrixes, one electronic and one opticalswitch matrix. The electronic switching matrix is to a great extentsimilar to today's electronic switches switching matrix, as known fromthe Prior Art, and works together with the control entity in a mannerknown from Prior Art “Best Effort” switches.

The optical switching matrix is supposed to function as a “wavelengthrouter”. An inward wavelength is sent to an appointed fibre and outwardwavelength. The outward wavelength and the outward fibre are fixedaccording to the inward wavelength and the inward fibre. As shown inFIG. 8 in the illustration only one fibre is used. This approach hassimilarities with MPLS wherein the wavelength can be considered as alabel.

At the input the signal is split according to the optical signal'spolarization. The advantage of this solution is a pure optical entitywhich may be used to split the traffic which is supposed to be handledas first priority, and which is supposed to be handled as BE-traffic.The solution presumes that the transmitting party is classifyingpriority traffic and BE traffic by sending these with orthogonalpolarization relatively to each other. The solution may in principle bean addition to the electronic switch wherein the electronic switch ismaintained as it is today, and handled “Best effort”-traffic, while theoptical wavelength router is handling first priority traffic. The designwill then differ from and be less optimal than shown in FIG. 8. Therewill be no possibilities for the electronic switch to let the opticalswitch handle the traffic when the latter has available capacity. Theutilization of the optical part will therefore become less optimal;however the design becomes considerably simplified.

In FIG. 8 it is thought that a number of wavelengths reserves to theelectronic switch and a number to the optical switch. If the opticalswitch is an addition, this number of wavelengths may be a fixed number,or it may be controlled centrally. If the two switches are builttogether from the beginning, the number of wavelengths can easier varyand be dynamic and be handled internally in the switch.

Optical Switching of Both GS and BE Packages with a Possibility ofElectronic Buffering.

Another design example is given in FIG. 9. By always delaying packetswith high priority (QoS packet), outputs can be reserved so that thedestined output is vacant when the packet occurs at the output of theoptical buffer.

In this design BE packets may choose freely among all the wavelengths atthe output of the switch. In the first stage of the switch the outputwavelengths is chosen. The QoS packets bypasses this first stage, hencethe QoS packets input wavelength will decide the output wavelength atthe output fibre. To which output fibre the packet is forwarded, isdecided by the wavelength set by the packet switch's second row ofwavelength-converters. In the third row of wavelength-converters, thewavelength will be set to a fixed wavelength matching the inputwavelength of specific input of a mux. Hence there is not possiblechanging the wavelength.

Because there is not possible to buffer the QoS packets for contentionresolution in this design, a reservation scheme will be necessary.Before a QoS packet is transmitted, a wavelength from the transmitter tothe destination should be reserved. This implies that each of thereceivers will simultaneously be able to receive QoS packets from anumber of destinations limited to the number of input wavelengths at thereceiver end. To avoid contention, no more than one source will beallowed to generate packets to each of the input wavelengths in a node.FIG. 10 illustrates this principle.

In the network shown in FIG. 10, when QoS information is sent from onenode to another, say from node 4, to node 6, a path consisting of one ormore wavelengths needs to be reserved all the way through the network,from node 4 to node 6. The reservation implies that no other nodes cantransmit QoS information at the reserved wavelength if the wavelengthsare sharing the same path (fibre) along the way. If two nodes wheretransmitting at the same wavelength, contention would occur. Since nobuffering is available for QoS packets, packets would have to bedropped. This can be avoided using reservation of a wavelength.

Wavelengths can both be reused, and added/dropped. As shown in the FIG.10, at node N1, the incoming wavelength from node N2, named N2-λ 4, isdropped at N1, and reused for sending QoS packets from node N1 to nodeN6 (N1-λ 4).

Scalability

Normally the node degree in a transport network like this will be in theorder of say 4-8. Also the total number of nodes in the transportnetwork will be limited. In COST 266 a reference scenario for a panEuropean network is given. FIG. 11 illustrates this network. In thenetwork a total of 37 Nodes is present, and it has a maximum node degreeof 5. The question is whether a static configuration of QoS resourceswill be sufficiently effective in a transport network. Whether this istrue will depend on the amount of QoS traffic in the network, and thenumber of wavelengths in each node. The packet switch design describedwill be effective only when a high number of wavelengths is available.This is because the design relies on using the wavelength dimension forcontention resolution. Number of wavelengths should therefore be 32 ormore

Dynamic Wavelength Allocation for Scalability

If a dynamic wavelength allocation scheme is employed, wavelengths candynamically be set up and taken down on demand. This will increase theutilization of the resources available for transmitting QoS packets,since it will allow dynamic changes in the traffic load. If the node inFIG. 9 is slightly modified, replacing the fixed wavelength convertersat the output with tuneable wavelength converters, the wavelength for aQoS path can be allowed to change along the way. This will allow ahigher reuse factor of the wavelengths in the network. However, atechnical problem when multiplexing the unpredictable wavelengths at theoutput of the tuneable wavelength converters has to be solved. Normallya low loss multiplexer will be wavelength sensitive. Other approaches toswitching the wavelength in the node may be evaluated.

Transparent Dynamic Line Switching and/or Burst Switching

The QoS packets do actually not need to be packets. They can be burstsof packets, i.e. burst switching can be performed, or it can be asemi-permanent line, i.e. line switching. It will all depend on thepreferred approach. When a QoS packet arrives at the input of theswitch, a change in the state of polarisation will be detected. Hence itis known that it is a QoS packet. At the end of a QoS packet, the stateof polarisation has to be changed back to is the “best effort state” sothat when the QoS packet has passed the polarisation monitor, the switchwill know the end of the packet. The output and the resources in theswitch will then be freed, so that resources can be used by the BestEffort packets.

Since the start and stop of the QoS packets is detected only by changein state of polarisation, and QoS packets will never be passed to thebuffer, there is no need to know the content of the information in theQoS packets. The physical format of the QoS packet, like bitrate andmodulation, can therefore in principle be varied. The limitations on thetransparency will be set by the characteristics of the wavelengthconverters.

For forwarding of the QoS packets, the destination of the packet must beknown in advance. This is for the switch to be able to set thewavelength converter in the second row of converters. The informationabout the QoS packet should be sent in advance. There are two approachesto this:

4a) Burst switching approach: This principle should be used if there arefew QoS paths in the network, and they will have to be reused often. QoSpaths can be established for a very short period of time, correspondingto the length of a packet, or a burst of packets.

The information about the destination for QoS packets occurring at aspecific input and wavelength is sent as a control packet. The controlpacket can be sent on a separate wavelength, or at the same wavelength,and will contain a header, telling the destination of the QoS packet(s).It does not need to contain information about the length of the packetor burst of packets, since the state of polarisation will tell the startand stop of the packets or burst. In burst switching, information aboutthe arrival time of the packet is sent out in advance. This is for theimmediate nodes to be able to reserve bandwidth during a, in the controlpacket, specified period of time. However since the QoS packets arealways buffered in the switch in an optical fibre delay line, bandwidthreservation is not necessary in advance, it will be done when the QoSpacket arrives at the switch. In addition, a protocol for distributionof the forwarding information will be needed. A table containing amapping between the address information in the control packet and how toset the wavelength converters is necessary.

4b) Line switching approach: This principle should be used when QoSpaths will have a duration that is much longer than a burst of packets.The output wavelength of the wavelength converter will be set to awavelength according to a table. The table will be updated by a protocoldistributing the forwarding information. This implies that no addresslookup will be needed when a QoS packet arrives, avoiding processing.However, there will be a mapping between a specified input (wavelength),and a specified output wavelength, that can be changed only by updatingthe table. The speed of the dynamic allocation of QoS paths willtherefore be limited by the protocol updating of the table.

Utilization of the QoS Paths

The node design in FIG. 9 allows the QoS paths to be utilized by BEpackets when QoS packets are not present. The lights state ofpolarisation is used for differencing between QoS and BE packets.Therefore, most of the capacity not used by the QoS traffic can be usedby the BE packets by interleaving these packets in between the QoSpackets or bursts. Technically, it is possible to transmit both QoS andBE packets simultaneously when the polarization is orthogonal. Thisimplies doubling the capacity in the fibre; however this also implies anumber of technical challenges with respect to the transmission qualityof the signal, since interference between the signals in the two statesof polarisation will occur.

When the node design in FIG. 9 is used, BE packets can be buffered andallocated a random wavelength along the path to its destination. Thisallows the capacity of the wavelengths, and also the wavelengths orpaths reserved for transmission of QoS packets, to be efficientlyutilized. When no QoS packet is present at a reserved QoS path, a BEpacket can, if it is available at one of the inputs or in the buffer, beswitched to the reserved QoS path.

When a QoS packet occurs at the input of the switch (node), the packetwill be sent in to an optical buffer with a delay corresponding to thelength of the longest BE packet. While the QoS packet is in the buffer,the reserved QoS path at the output of the switch will be left vacantfrom the time when the last packet at this output has left the outputuntil the QoS packet reaches the output of the buffer. The opticalbuffer will give a predictable delay, thus causing no jitter, with amagnitude insignificant compared to the transmission delay in the fibrebetween the nodes.

Abrreviation list AWG Arrayed Waveguide Gratings, a component using in-terferometer principles for wavelength routing or MUX/DEMUX. BE: Besteffort BER Bit Error Rate Cw Continuous Wave, laser emitting light, notmodu- lated. D-WRON Dynamic Wavelength Routed Optical Network EDFAErbium-Doped Fibre Amplifier A form of fibre opti- cal amplification inwhich the transmitted light signal passes through a section erbium-dopedfibre and is amplified by means of a laser pump diode. EDFA is used intransmitter booster amplifiers, in-line repeating amplifiers, andreceiver pream- plifiers. FDL Fibre Delay Line. FSK Frequency shiftkeying. A modulation technique for data transmission. It shifts thefrequency above the carrier for a 1 and below the carrier for a 0 (zero)GMPLS Generalized Multiprotocol Label Switching GS Guaranteed service.IST EUs 5th program for support of a number of proj- ects. MPLSMultiprotocol label switching, this is a “multi- protocol forwardingstandard” describing a method for integration of IP and ATM throughlabel swap- ping. A IP- and ATM switch, i.e. a MPLS switch, comprisesATM hardware with MPLS software. The software is IP addressing,IP-routing and label distribution protocol. NRZ Non Return to Zero. OCSOptical circuit switched (networks). OPS Optical packet switched(networks). OTDM Optical time division multiplexing. PBS PolarisationBeam Splitter. PC Polarisation Controller PLR Packet Loss Ratio PMPolarizing Maintaining. PMD Polarization Mode Dispersion. A dispersioneffect created by irregularities in the shape of the fi- bre optic cableand it score. Resulting in light propagation at different speeds in thetwo differ- ent modes of polarisation PRBS Pseudo Random BitSequence/pattern. A test pattern having the properties of random data(generally 511 or 2047 bits), but generated in such manner that anothercircuit operating independently, can synchronize on the pattern anddetect individual transmission bit errors. QoS Quality of service. RZReturn to Zero. SCM Sub Carrier Modulation combines a signal with asingle low frequency sine wave. The low frequency signal is called asubcarrier. This combined sig- nal is then added to a higher frequencyradio sig- nal. The resulting high frequency radio signal is verycomplex and the original signal is not de- tectable by ordinary means.To detect a signal that has been modulated by a subcarrier, it must bepassed through two detector circuits, one to separate the subcarrierfrom the high frequency radio transmission, and a second to separate thesubcarrier from the desired information. SMF Single mode Fibre. SOASemiconductor Optical Amplifier. SOP States Of Polarisation. S-WRONStatic Wavelength Routed Optical Network. WDM Optical WavelengthDivision Multiplexing

REFERENCES

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1. A communication network arrangement for handling packets withinoptical or combined optical/electrical packet switched networks, thecommunication network arrangement comprising: means for dividing packetswithin the network by first and second QoS classes; means fortransmitting packets of the first QoS class in a first state ofpolarization and transmitting packets of the second QoS in a secondstate of polarization; an ingress node; and at least one core node, saidcore node having: at least one polarisation beam splitter (PBS1), twooptical demultiplexers, at least one first wavelength converter, asecond wavelength router, and at least one third fixed wavelengthconverter adapted to forward packets of the first and second QoS classto a first optical multiplexer.
 2. A communication network arrangementfor handling packets within optical or combined optical/electricalpacket switched networks comprising: means for dividing packets withinthe network by first and second QoS classes; means for transmittingpackets of the first QoS class in a first state of polarization andtransmitting packets of the second QoS in a second state ofpolarization; an ingress node, wherein said ingress node: has means forseparating header and payload for BE-packets by state of polarisation,and has means for separating packets by changing state of polarisationat the beginning of every new packet, using at least one polarisationbeam splitter (PBS) adapted to receive a WDM-signal with a plurality ofwavelengths and wherein the polarisation beam splitter (PBS) is adaptedto separate header and payload by using the polarisation beam splitterper wavelength; and at least one core node, said core node having atleast one polarisation beam splitter (PBS1) and at least one opticaldemultiplexer.
 3. A communication network arrangement for handlingpackets within optical or combined optical/electrical packet switchednetworks, the communication network arrangement comprising: means fordividing packets within the network by first and second QoS classes;means for transmitting packets of the first QoS class in a first stateof polarization and transmitting packets of the second QoS in a secondstate of polarization; an ingress node; and at least one core node, saidcore node having at least one polarisation beam splitter (PBS1) and atleast one optical demultiplexer, wherein the ingress node and the atleast one core node comprises an optical packet switched module attachedto a S-WRON node.
 4. A method for handling packets within optical orcombined optical/electrical packet switched networks comprising at leastan ingress node for multiplexing of optical packets by polarization, andan egress node for demultiplexing of received optical packets bypolarization comprising: dividing packets of the ingress node as firstand second QoS classes of packets, and transmitting packets of the firstQoS class in a first state of polarization and transmitting packets ofthe second QoS in a second state of polarization, by either interleavingpackets of the second QoS class of packets with packets of the first QoSclass or by simultaneously transmitting packets of a first QoS class ina first state of polarization and transmitting packets of a second QoSclass in a second state of polarization, the states of polarizationbeing substantially orthogonal, wherein the network further has at leastone core node that executes at least one of the following steps: a)demultiplexing received traffic by polarisation, b) polarizing thereceived traffic, and c) SOP-aligning received traffic.
 5. The method ofclaim 4, wherein at least one core node in the optical packet switchednetwork is SOP-realigning received packets.
 6. The method of claim 4,wherein when a first packet of a first QoS class arrives at a switch thefollowing steps are carried out: a controlling device registering thatthe first packet is present at the input, then delaying the first packetin a FDL in a first pre-determined period of time, and reserving anoutput where the first packet is directed to be transmitted, andcommunicating the first packet exiting a FDL to a reserved vacantoutput.
 7. The method of claim 6, further comprising defining the firstpredefined period of time to be longer than a second period of time, anddefining the second period of time using a packet with a lower QoS levelthan the first packet where the second packet is of a maximum allowedsize.
 8. A method for handling packets within optical or combinedoptical/electrical packet switched networks comprising at least aningress node for multiplexing of optical packets by polarization, anegress node for demultiplexing of received optical packets bypolarization, and at least one core node, comprising: dividing packetsof the ingress node as first and second QoS classes of packets, andtransmitting packets of the first QoS class in a first state ofpolarization and transmitting packets of the second QoS in a secondstate of polarization, by either interleaving packets of the second QoSclass of packets with packets of the first QoS class or bysimultaneously transmitting packets of a first QoS class in a firststate of polarization and transmitting packets of a second QoS class ina second state of polarization, the states of polarization beingsubstantially orthogonal; and interchanging said first and said secondstates of polarization at the beginning of each packet, wherein the atleast one core node executes time divisional multiplexing of receivedpackets.
 9. A method for handling packets within optical or combinedoptical/electrical packet switched networks comprising at least aningress node for multiplexing of optical packets by polarization and anegress node for demultiplexing of received optical packets bypolarization, comprising: dividing packets of the ingress node as firstand second QoS classes of packets, and transmitting packets of the firstQoS class in a first state of polarization and transmitting packets ofthe second QoS in a second state of polarization, wherein the second andfirst state of polarization are substantially orthogonal states, byeither interleaving packets of the second QoS class of packets withpackets of the first QoS class or by simultaneously transmitting packetsof a first QoS class in a first state of polarization and transmittingpackets of a second QoS class in a second state of polarization, thestates of polarization being substantially orthogonal, whereinstatistically multiplexed packets of the second QoS class areinterleaved with packets of the first QoS class, and wherein the packetsof the first QoS class use a predefined wavelength path within acommunication network.
 10. A method for handling packets within opticalor combined optical/electrical packet switched networks comprising atleast an ingress node for multiplexing of optical packets bypolarization and an egress node for demultiplexing of received opticalpackets by polarization, comprising: dividing packets of the ingressnode as first and second QoS classes of packets; transmitting packets ofthe first QoS class in a first state of polarization and transmittingpackets of the second QoS in a second state of polarization, by eitherinterleaving packets of the second QoS class of packets with packets ofthe first QoS class or by simultaneously transmitting packets of a firstQoS class in a first state of polarization and transmitting packets of asecond QoS class in a second state of polarization, the states ofpolarization being substantially orthogronal; assigning the first QoSclass to GS-packets and assigning the second QoS class to BE-packets;and forwarding GS-packets optically using an optical switch andforwarding BE-packets electronically using an electronic switch.
 11. Acommunication network arrangement for handling packets within optical orcombined optical/electrical packet switched networks comprising: meansfor dividing packets within the network by first and second QoS classes;means for transmitting packets of the first QoS class in a first stateof polarization and transmitting packets of the second QoS in a secondstate of polarization; an ingress node, wherein said ingress node: hasmeans for separating header and payload for BE-packets by state ofpolarisation, and has means for separating packets by changing state ofpolarisation at the beginning of every new packet, using at least onepolarisation beam splitter and at least one optical demultiplexeradapted to receive a WDM-signal with a plurality of wavelengths, whereinthe ingress node is adapted to separate header and payload by using thepolarisation beam splitter per wavelength; and at least one core node,said core node having at least one polarisation beam splitter and atleast one optical demultiplexer.